The present invention relates in general to output stages for logic and pulse circuits and in particular to an output stage providing a differential output signal that switches between predetermined high and low levels in an accurately controlled fashion.
Logic and pulse circuits often produce output signals for controlling other circuits but which are unsuitable for applying directly as inputs to the other circuits. For example, output signals produced by a control circuit may swing between levels that differ from the levels recognized by a circuit to be controlled, or the output signals may not convey sufficient power to drive the controlled circuit. In such case, an output stage is typically provided to convert an output signal produced by the control circuit to an output signal that is more appropriate for use as a circuit input.
A differential output stage produces a differential output voltage signal which may be thought of as a potential between two "single-ended" output voltage signals that are referenced to ground or other point of common reference potential. For example, a differential voltage signal suitable for application as input to emitter-coupled logic (ECL) circuits swings between a "high" level of +1 volt and a "low" level -1 volt to indicate a state change. Such an ECL differential voltage signal is suitably a potential between two single-ended voltage signals that swing in opposite directions between a high level of -0.8 volts and a low level of -1.8 volts, referenced to ground. Typically, an ECL logic circuit will recognize a differential input signal as representing a change in logic state when one of the single-ended voltage signals that form a differential input signal rises above -1.3 volts (the midpoint of its swing) and the other single-ended signal falls below -1.3 volts. Consequently, when the two single-ended voltage signals cross the -1.3 volt midpoint level at different times, the logic state represented by the differential input signal is indeterminate for the period of time between the two midpoint crossings. In some applications, the behavior of a circuit receiving the differential signal as input can be unpredictable during such period and can lead to undesirable results. Also, many prior art circuits utilizing field effect transistors do not have well-controlled high and low output levels.